1. Field of the Invention
The present invention relates to an image forming apparatus having a process for developing an electrostatic image using two-component developer having toner and carrier. More specifically, the present invention relates to an image forming apparatus that forms an image using colored toners and transparent toner.
2. Description of the Related Art
For example, in the field of electrophotographic image forming apparatus, there is widely used a color image forming apparatus that forms a multicolor image such as a full-color image using a plurality of colors of toner.
Conventional image forming apparatuses use four colors of cyan (C), magenta (M), yellow (Y), and black (BK). With the recent advancement of image forming apparatuses, needs have increased, and image forming apparatuses using an increased number of colors have been proposed. Some of them use light colors such as light cyan and light magenta, which are commonly used in ink jet image forming apparatuses, in addition to the conventional four colors. Japanese Patent Laid-Open No. 8-220821 discloses using transparent toner in addition to the four colors of toner.
The main purpose to add transparent toner is to uniformize the gloss in the image plane. In the case of electrophotographic image formation, a difference in level in the image plane due to the difference in the amount of colored toners causes unevenness of gloss in the image plane. Putting transparent toner so that the amount of transparent toner in a unit area is inversely proportional to the amount of colored toners in the unit area eliminates the difference in level in the image plane and uniformizes the gloss.
There are various types of image forming apparatuses having an increased number of colors of developer. Tandem-type image forming apparatuses form an image using the same number of image bearing members (photosensitive members) as the number of kinds of toners. A tandem-type image forming apparatus using, for example, six colors of developer (toner) includes six tandemly-arranged image forming units. Each image forming unit includes a combination of an image bearing member and a developing device. Each developing device is loaded with developer having a different spectral characteristic.
Known developing methods utilized in electrophotographic methods include a two-component developing method using two-component developer including non-magnetic toner particles (toner) and magnetic particles (carrier) and a one-component developing method that does not use carrier. In the above-described image-quality-oriented image forming apparatuses using light-colored toners or transparent toner, the two-component developing method is often used from the viewpoint of high-resolution and stability of the amount of toner per unit area.
In the two-component developing method, the image on the image bearing member tends to be affected by friction with the magnetic brush of carrier, and image deterioration such as worsening of graininess in a low-density part can occur. A developing method in which the rotational direction of the developer bearing member is opposite to the rotational direction of the image bearing member in the developing region will be referred to as counter developing method. A developing method in which the rotational direction of the developer bearing member is the same as the rotational direction of the image bearing member in the developing region will be referred to as forward developing method. The frictional force between the developer and the image bearing member in the counter developing method is larger than that in the forward developing method. Therefore, graininess worsens significantly. Therefore, in image-quality-oriented image forming apparatuses, the forward developing method is often used to reduce graininess.
However, when the rotational direction of the developer bearing member is the same as the rotational direction of the image bearing member in the developing region, there is concern over a phenomenon in which when a high-density part is behind a low-density part, the rear end of the low-density part decreases in density (missing image) (see FIG. 4).
In many cases, the rotational speed of the image bearing member differs from the rotational speed of the developer bearing member, and the rotational speed of the image bearing member and the rotational speed of the developer bearing member are in the ratio of 1:1 to 1:3. If the rotational speed of the developer bearing member is less than or equal to the rotational speed of the image bearing member, the amount of developer supplied in the developing region is insufficient and therefore a sufficient image density cannot be obtained. Therefore, generally, the rotational speed of the developer bearing member is larger than the rotational speed of the image bearing member. In this case, the following phenomenon occurs around the developing region (FIGS. 5A and 5B). FIG. 5A shows a latent image entering the developing region, the latent image including a high-density part and a low-density part therebehind. In the developing region, toner moves to the image bearing member in accordance with the latent image. The electric charge of two-component developer is substantially zero when toner and carrier are in contact with each other. When the toner moves to the image bearing member and separates from the carrier, for example, in the case where the toner is negatively charged, the two-component developer left behind is positively charged (this phenomenon will hereinafter be referred to as counter charge). As a result, a part of the negative toner moved to the surface of the image bearing member is attracted by the positive charge generated in the two-component developer and re-adheres to the carrier (hereinafter referred to as missing image (FIG. 5B)). Such a missing image phenomenon occurs significantly at the rear end of a low-density part in front of a high-density part because the developer bearing member precedes due to the difference between the rotational speed of the image bearing member and the rotational speed of the developer bearing member.
It is known that the missing image phenomenon is reduced by lowering the resistance of carrier of the two-component developer. The mechanism will be described. As described above, the part of the negative toner moved to the surface of the image bearing member is attracted by the counter charge of the two-component developer and re-adheres to the carrier, thereby causing the missing image phenomenon. Therefore, the missing image is reduced by lowering the amount of counter charge. When the carrier has low resistance, the counter charge generated in the carrier escapes quickly through the sleeve and therefore the positive charge decreases. Therefore, the Coulomb's force between the carrier and the negative toner on the image bearing member decreases. Therefore, the missing image is reduced. However, if the resistance of carrier is too low, the carrier adheres to the image bearing member or the latent image on the image bearing member is disturbed.
A method in which the rotational direction of the developer bearing member is opposite to the rotational direction of the image bearing member in the developing region, that is, a counter developing method can be used to prevent the missing image from occurring. However, as described above, since the difference in speed between the image bearing member and the developer bearing member is larger and the friction force to which the image bearing member is subjected is larger as compared to the forward developing method, the graininess worsens.
Therefore, in conventional four-color (yellow, magenta, cyan, and black) image forming apparatuses, the graininess is reduced using the forward developing method, and the missing image phenomenon is prevented by not lowering the resistance of carrier to the level where the carrier adhesion occurs.
However, image forming apparatuses that are intended to eliminate the difference in toner level in the image plane to uniformize the gloss and that use a plurality of colored toners and transparent toner have the following problem. When the forward developing method is used in every developing device, the missing image phenomenon occurs more significantly in the transparent toner developing device than in the colored toner developing devices. The main cause thereof will be described.
In terms of the function of transparent toner, the maximum amount per unit area of transparent toner used for development in the transparent toner developing device needs to be substantially equal to the total maximum amount of toners used for development in all colored toner developing devices. The reason is that the transparent toner is used to planarize the toner image in the image plane and to thereby uniformize the gloss. To make up the difference between the total maximum amount of colored toners per unit area and the minimum amount (blank part), the amount of transparent toner needs to be equal to the total maximum amount of colored toners. Therefore, for example, when the total maximum amount of toner per unit area in yellow, magenta, cyan, and black toner developing devices is 200% (when the maximum amount of one color of toner per unit area is 100%), the transparent toner developing device needs to use up to twice the amount of toner used in each colored toner developing device.
The missing image phenomenon tends to become more significant with the increase in the maximum amount of toner per unit area under a condition in which the charge amount of toner per unit mass is equal. With the increase in the amount of toner per unit area, more negative toner moves from the two-component developer to the image bearing member, and therefore the positive charge amount of the two-component developer left behind increases. As a result, the Coulomb's force between the negative toner on the image bearing member and the two-component developer increases, and therefore the missing image worsens. In the case of transparent toner, the part of the missing image does not decrease in density unlike in the case of colored toner. However, since the part of the missing image is significantly different in gloss (generally deteriorates in gloss), the uniformity in gloss in the image plane, which is the purpose of use of transparent toner, is diminished.
This worsening of the missing image due to the increase in the amount of toner per unit area can be prevented by lowering the charge amount of toner per unit mass. Since the charge amount of toner per unit mass is lowered, if the amount of toner per unit area increases, the amount of positive charge of the two-component developer generated after development does not increase, and therefore the missing image phenomenon can be restrained. However, if the charge amount of toner per unit mass is too small, the toner scatters outside the latent image due to the centrifugal force due to the rotation of the developer bearing member. This problem is called toner scattering. The maximum amount of transparent toner per unit area needs to be about 200 to 300% of the maximum amount of colored toner per unit area in actual use. It is virtually impossible to lower the charge amount of transparent toner to ½ to ⅓ in consideration of the above-described toner scattering.
As described above, when the forward developing method is used in every developing device, the missing image phenomenon is more significant in the transparent toner developing device than in the colored toner developing devices. In the case of transparent toner, the part of the missing image does not decrease in density unlike in the case of colored toner. However, since the part of the missing image is different in gloss, the uniformity in gloss in the image plane is diminished.